Chokes are commonly used in electronic circuits to block signal frequencies above a desired range, while at the same time allowing DC or low frequency signals to pass. Thus, chokes have been employed to prevent electromagnetic interference (EMI) from disturbing various electronic devices. EMI is generated, for example, as a byproduct of switching regulators that have current and voltage waveforms with fast rise and fall times. Because switching regulators are typically contained in power supplies, EMI may be transmitted through an electronic device via the power supply conductors. Excessive EMI can lead to logic errors in a computer and can cause interference with other adjacent electronic components. Of course, there are many other applications where a choke may be needed to filter unwanted signals.
A choke is typically provided by a magnetic core through which, or around which, conductors or windings are positioned. Thus, a typical choke defines first and second mutually coupled magnetic paths. A choke may be schematically represented as a low pass filter. For any choke to function as intended, its inductance or inductive reactance, should not fall below a specific minimum, even though the current in a winding rises to a maximum value. Beyond the maximum current value, the reactance falls off significantly. The choke's ability to impede interference signals drops, thereby allowing the passage of unwanted signals. It is therefore typically desirable to prevent a choke from being driven into such a saturation condition.
Ferrite materials are commonly used as the core material for many chokes because, for example, ferrites have sensitive magnetic-frequency relationships. The ferrite material used to form the choke will determine which signal frequencies the choke will attenuate. Most ferrites having suitable inductance values for choke applications saturate at less than about 4,000 Gauss. Accordingly, when configured differentially, ferrites have a relatively low current carrying capacity before the choke is driven into saturation and its impedance level deteriorates at the desired filter frequency.
The techniques normally used to prevent this saturation are to provide a core air gap, use a larger cross-section core, or simply to limit the allowable current. An example of a choke with a core having an air gap is illustrated in U.S. Pat. No. 5,455,552, assigned to the assignee of the present invention. The choke represents a significant advance in technology and includes a ferrite body through which a pair of spaced apart vertical slotted openings are formed. Pairs of spaced apart electrical conductors extend from the bottom of the device, vertically to the top of the device through one slot, along the top of the device, and back down vertically through the other slot of the device. The conductors are configured for common mode operation to mitigate the saturation difficulties. In other words, by bringing the high side and ground return through the same core opposing fields are produced in the core which tend to cancel.
While chokes are commonly applied directly to printed circuit boards, in such applications it is typically undesirable to use chokes with large cores or gapped sections. Accordingly, the compact ferrite structure as disclosed in U.S. Pat. No. 5,455,552 is advantageous for circuit board mounting. Unfortunately, the physical positioning of the discrete conductors through the vertically extending slots requires additional efforts during manufacturing.
A further advance in the area of ferrite chokes is disclosed in U.S. Pat. No. 6,288,626, assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference. This patent discloses a punching operation to provide an air gap between the parallel conductors within a ferrite body. The choke, however, requires a relatively large footprint for a given electrical performance.